Union heads representing thousands of America's immigration agents slammed the Obama administration Thursday over a policy they claim is forcing officials to ignore the law and allowing illegal immigrants to exploit the system.

In a startling allegation, the president of the union representing Immigration and Customs Enforcement officers claimed illegal immigrants are "taking advantage" of a new directive allowing some undocumented residents who came to the U.S. as children to stay in the country. Union boss Chris Crane said the policy ends up allowing illegal immigrants to avoid detention without any proof -- particularly so-called "dreamers," or those illegal immigrants who would benefit under the "DREAM Act" proposal, which Congress has not passed but the administration has partially implemented.

"Prosecutorial discretion for dreamers is solely based on the individual's claims. Our orders are if an alien says they went to high school, then let them go," he said at a press conference with GOP senators. "Officers have been told that there is no burden for the alien to prove anything. ... At this point we don't even know why DHS has criteria at all, as there is no requirement or burden to prove anything on the part of the alien.

"We believe that significant numbers of people who are not dreamers are taking advantage of this practice to avoid arrest," he said.

Crane cited one case in which, he said, an immigrant facing criminal charges was let go under the policy. Further, he complained that officers are "under threat of losing their jobs" if they defy the policy.

ICE didn't respond to those allegations directly but said allegations of fraud and abuse will be investigated.

The press conference comes just days after the Federation for American Immigration Reform, an anti-illegal immigration group, released a report claiming the administration has worked for the past three-and-a-half years to undermine immigration enforcement.

ICE, though, defended its operations Thursday evening -- noting, as it often does, that the agency is only funded to remove roughly 400,000 people a year and has to prioritize. The prioritization under this administration has been to fast-track for deportation those accused of serious criminal offenses and potentially give a reprieve to those who aren't.

"Because the agency encounters more removable aliens than it is able to process, the agency prioritizes its resources on aliens whose removal have the greatest impact on public safety and the integrity of the immigration system," spokeswoman Gillian Christensen said in a statement. "ICE's efforts to target the agency's enforcement priorities are paying big dividends. For each of the past three fiscal years, ICE has removed more criminal aliens from the country than ever before and more than 90 percent of ICE's removals fell into one of the agency's enforcement priority categories."

She disputed the notion that the policy impacts public safety, but she said the agency meets regularly with union representatives "to discuss our goal of ensuring public safety by focusing on the removal of individuals who meet our enforcement priorities."

In light of the accusations Thursday, she added that "the agency will aggressively investigate allegations of fraud and abuse of the deferred action process."

The allegations from the union were expressed in unusually blunt terms Thursday.

George McCubbin, president of the National Border Patrol Council union, said the Department of Homeland Security has made it impossible for agents to do their jobs.

Crane said it's led to disorganization and "confusion" at ICE.

Republican lawmakers used the forum to continue their sustained campaign against the administration's immigration policy changes.

Sen. Charles Grassley, R-Iowa, said the latest directive is without legal basis and "condones breaking the law."